Here's why:

* Mineral Composition: Mafic rocks are primarily composed of minerals like pyroxene and olivine, which are rich in magnesium (Mg) and iron (Fe). These minerals are denser than the minerals found in felsic rocks. Felsic rocks are dominated by minerals like quartz and feldspar, which are richer in silica (SiO2) and have a lower density.

* Chemical Composition: Mafic rocks have a higher concentration of heavier elements like magnesium, iron, and calcium, while felsic rocks are richer in lighter elements like silicon, oxygen, and aluminum. This difference in chemical composition contributes to the density variation.

To put it simply: Mafic rocks are like the "heavyweight" rocks, while felsic rocks are the "lightweight" rocks.